The invention relates to an exhaust gas purifying apparatus for internal combustion engines.
Heretofore, various proposals have been made for an exhaust gas purifying apparatus for internal combustion engines, an example thereof being disclosed in Japanese Patent Laid-Open No. 106826/1987 and Japanese Patent No. 2600492.
Japanese Patent Laid-Open No. 106826/1987 discloses an arrangement, in which an engine exhaust passage is branched into a pair of exhaust branch passages, two NOx adsorbing catalysts of the same dimension are arranged in parallel in the respective branch passages, and a switchover valve is provided in a branch of the exhaust passage. The switchover valve prevents exhaust gases from passing through the catalyst in one of the exhaust branch passages, and exhaust gases are caused to pass through one of the NOx adsorbing catalysts to adsorb NOx while NOx accumulated in the other of the NOx adsorbing catalysts is reduced and removed by means of a gaseous reducing agent. In addition, for example, hydrocarbon is used as the gaseous reducing agent and introduced from a hydrocarbon generating apparatus.
Japanese Patent No. 2600492 discloses an arrangement, in which a single NOx adsorbent is arranged in an exhaust passage of an engine and constantly passes therethrough exhaust gases in operation of the engine. In normal operation, an air-fuel mixture in engine cylinders is lean and NOx in exhaust gases is adsorbed by the NOx adsorbent. When a NOx accumulation in the NOx adsorbent is increased in amount, an oxygen concentration in exhaust gases is decreased to release NOx from the NOx adsorbent and reduce the same. In order to decrease an oxygen concentration in exhaust gases, an air-fuel ratio of exhaust gases is made substantially stoichiometric air-fuel ratio or rich.
However, the above-mentioned arrangements involve the following problems.
With the arrangement disclosed in Japanese Patent Laid-Open No. 106826/1987, the two same NOx adsorbing catalysts are arranged in parallel to become large in space and volume, which makes it difficult for such arrangement to be mounted on, for example, a vehicle or the like. Also, the use of hydrocarbon being a gaseous reducing agent causes a need of a hydrocarbon generating apparatus, which makes the construction complicated and increases cost.
With the arrangement disclosed in Japanese Patent No. 2600492, an amount of fuel supplied into cylinders is increased when exhaust gases are made rich in air-fuel ratio in order to release NOx from the NOx adsorbent and reduce the same. Therefore, an increase in exhaust temperature and cylinder pressure is caused at the time of high output, which causes a fear of adverse influences on durability and reliability of an engine.
The invention has been thought of in view of the above-mentioned problems, and has its object to provide an exhaust gas purifying apparatus for internal combustion engines, which is decreased in space and volume, free of adverse influences on durability and reliability of an engine, simple in construction, and low in cost.
In order to attain the above-mentioned object, a first invention provides an exhaust gas purifying apparatus for internal combustion engines, in which a NOx adsorbing/reducing catalyst device for adsorbing NOx and decreasing an oxygen concentration to feed a reducing agent to release and reduce NOx when exhaust gases are lean in air-fuel ratio is provided in an exhaust pipe of an internal combustion engine, the apparatus comprising a first NOx adsorbing catalyst, a second NOx adsorbing catalyst provided in parallel to the first NOx adsorbing catalyst and having a small adsorbing capacity as compared with that of the first NOx adsorbing catalyst, an exhaust gas flow passage switchover means for switching an exhaust gas flow between the first NOx adsorbing catalyst and the second NOx adsorbing catalyst, first NOx releasing and reducing means for causing a gas containing a reducing agent to flow toward the first NOx adsorbing catalyst to decrease an oxygen concentration and releasing and reducing NOx while exhaust gases flow toward the second NOx adsorbing catalyst, and second NOx releasing and reducing means for causing a gas containing a reducing agent to flow toward the second NOx adsorbing catalyst to decrease an oxygen concentration and releasing and reducing NOx while exhaust gases flow toward the first NOx adsorbing catalyst.
According to the first invention, two NOx adsorbing/reducing catalyst devices having different capacities are provided in parallel, and flow of exhaust gases directed to the NOx adsorbing catalysts is switched over to one of the catalysts by the exhaust gas flow passage switchover means. For example, a small amount of exhaust gases or a gas such as air or the like is caused to flow into a side, in which flow of exhaust gases is stopped, to generate flow passing through the NOx adsorbing catalyst, and for example, a fuel is fed as a reducing agent to enable decreasing an oxygen concentration and releasing and reducing NOx. Therefore, even when NOx is released and reduced at the time of high output, degradation in durability and reliability of the engine due to an increase in exhaust temperature and cylinder pressure is prevented since any surplus load is not supplied into cylinders. Also, since it suffices that a small amount of exhaust gases or air be made rich, an amount of a reducing agent can be decreased and the apparatus can be made small in space and volume and inexpensive because of one of the NOx adsorbing catalysts being made smaller in capacity than the other.
A second invention comprises, in addition to the constitution of the first invention, exhaust gas passing means, by which a smaller amount of exhaust gases than that of exhaust gases flowing toward the second NOx adsorbing catalyst is permitted to flow toward the first NOx adsorbing catalyst when NOx in the first NOx adsorbing catalyst is to be released and reduced while exhaust gases flow toward the second NOx adsorbing catalyst and a smaller amount of exhaust gases than that of exhaust gases flowing toward the first NOx adsorbing catalyst is permitted to flow toward the second NOx adsorbing catalyst when NOx in the second NOx adsorbing catalyst is to be released and reduced while exhaust gases flow toward the first NOx adsorbing catalyst, and reducing agent feeding means for making exhaust gases, which are on a side where NOx is to be released and reduced, near a stoichiometric mixture ratio or rich.
According to the second invention, a small amount of exhaust gases is caused to flow toward that NOx adsorbing catalyst, from which NOx is released and reduced, and a reducing agent is fed to the NOx adsorbing catalyst to make exhaust gases near a stoichiometric mixture ratio or rich, so that a small amount of the reducing agent serves in use. Also, since a gas for generating flow on that NOx adsorbing catalyst, from which NOx is released and reduced, is exhaust gases, the apparatus is made simple in construction and can be made inexpensive.
A third invention comprises, in addition to the constitution of the first and second inventions, NOx accumulation checking means for checking a NOx accumulation in the first NOx adsorbing catalyst, and exhaust gas flow passage switchover/control means for permitting exhaust gases to flow toward the first NOx adsorbing catalyst when the engine is normally operated, and permitting exhaust gases to flow toward the second NOx adsorbing catalyst when a NOx accumulation in the first NOx adsorbing catalyst exceeds a predetermined value.
According to the third invention, a NOx accumulation in the first NOx adsorbing catalyst is checked and flow toward the first or second NOx adsorbing catalyst is switched over, so that timing in switchover becomes correct and the number of switchover and an amount of a reducing agent used can be decreased, which makes the apparatus economical.
A fourth invention comprises, in addition to the constitution of the first and second inventions, NOx releasing/reducing control means, by which switchover of the exhaust gas flow passage caused by the exhaust gas flow passage switchover means, and NOx releasing/reducing operations caused by the first and second NOx releasing and reducing means are performed at predetermined time intervals.
According to the fourth invention, switchover of flow passages, release and reduction can be made periodically on the basis of periods of time, in which a NOx accumulation in the respective NOx adsorbing catalysts having been beforehand checked reaches allowable limits, and periods of time required for release and reduction of NOx. Therefore, NOx accumulation checking means or the like is dispensed with, and the apparatus is simple in construction and low in cost.
According to a fifth invention, the second NOx adsorbing catalyst has a capacity conformed to a period of time, in which NOx adsorbed by the first NOx adsorbing catalyst can be released and reduced, in the constitution of the first to fourth inventions.
According to the fifth invention, it is possible to attain optimization of the second NOx adsorbing catalyst in dimension and make the same minimum, thus enabling making the apparatus small in space and volume.
According to a sixth invention, the second NOx adsorbing catalyst has a capacity ⅕ to {fraction (1/20)} as much as that of the first NOx adsorbing catalyst in the constitution of the first to fifth inventions.
According to the sixth invention, since the NOx adsorbing catalyst can be made small in space and volume, the exhaust gas purifying apparatus can be made small in size.
According to a seventh invention, an amount of a gas flowing to that NOx adsorbing catalyst, from which NOx is to be released and reduced, is {fraction (1/10)} or less as much as a whole amount of exhaust gases in the constitution of the first to sixth inventions.
According to the seventh invention, exhaust gases flowing at the time of release and reduction of NOx are small in amount and made near a stoichiometric mixture ratio or rich, so that an amount of a reducing agent used can be decreased to lead to economy.